During the last century, humans have endured multiple severe viral outbreaks including the Spanish flu and other influenza epidemics, HIV, SARS-CoV, MERS, Ebola and the recent SARS-CoV2. During this period, science has made strides in understanding many different facets of virus outbreaks including viral infectivity, pathogenicity, and virulence, transmission, and propensity to cause an epidemic/pandemic.

Modern techniques that include cell culture, infectivity assays, polymerase chain reactions, fluorescence microscopy, electron microscopy, and immunoassays have helped us gain a better understanding about the virus life cycle, viral replication, and the virus-host interaction. We now have a better grasp on the structure and function of the viral particle, viral genome, RNA or DNA expression levels, virus-host cell interaction and immune response. These advances have resulted in the development of vaccines and drugs to treat several different viruses.

Atomic force microscopy (AFM) is one of the newer techniques available for virus research. AFM is a cantilever-based technique that utilizes a sharp tip to interrogate surfaces at resolutions well below the optical diffraction limit. Beyond imaging, AFM is also a powerful tool for nano-mechanical probing and nano-manipulation.

One of the primary advantages of AFM is that it can operate on samples immersed in liquid. This empowers experiments on living cells at physiologically relevant conditions. AFM imaging has been used to study live virus particles, to investigate the dimension and morphology and packaging of viral genomic material. Beyond imaging, AFM has been used for manipulation of single viruses by force spectroscopy to study early events of virus-host interactions.

Cooperative Vaccinia Infection

The first study aimed to study the cooperativity of virus particles in cell infection. In other words, to find out how the probability of virus infection depends on the amount of virus particles attacking a cell. To study cooperativity of virus infection, accurate control is needed on the timing of infection and the number of virions to which a cell is exposed.

Two pioneering labs, the Laboratory of Biosensors and Bioelectronics and the Institute of Microbiology of the ETH Zurich used the FluidFM® to precisely deliver a defined number of virus particles onto single cells.

Conclusion

AFM is a powerful technique to analyze and manipulate biological samples under physiological conditions. It can be combined with advanced optical techniques, to allow correlation of optical characterization of the sample cells in parallel to the AFM experiments.

In this application we showed two studies of host-virus interactions at the single cell level. In the first example the AFM was used as a manipulation tool using FluidFM technology. In the second example the mass development of virus-infected cells was studied after infection.

Virus research in the past has led to the development of treatments and vaccines. New viruses like the recent SARS-CoV2, which has led to the COVID-19 pandemic, stressing the importance of continued virus research to become a better and faster response to future outbreaks.